SQL 从 YTD 中计算出 MTD

Question

I try to calculate in SQL MTD cost of a table.

I have following table:

year	user	month	type	cost	blank
2021	a	1	type1	10	0
2021	a	2	type1	20	0
2021	a	3	type1	35	0
2022	a	1	type1	5	0
2022	a	2	type1	35	0
2021	b	1	type1	10	0
2021	b	2	type1	30	0

What I need to have is now the MTD cost as per year, user and type

year	user	month	type	cost	costMTD	blank
2021	a	1	type1	10	10	0
2021	a	2	type1	20	10	0
2021	a	3	type1	35	15	0
2022	a	1	type1	5	5	0
2022	a	2	type1	35	30	0
2021	b	1	type1	10	10	0
2021	b	2	type1	30	20	0

Can i do this with a query in SQL?

I tried like this but it doesn't work:

SELECT t1.year, t1.user, t1.month, t1.type, t1.cost, 
iif(t1.month = '1', t1.cost, t1.cost- t2.cost) AS costMTD, 0 AS blank
FROM dbo.usercosts AS t1 INNER JOIN dbo.usercosts AS t2 
ON t1.year = t2.year 
AND t1.user= t2.user 
AND t1.type= t2.type 
AND t2.month = iif(t2.month = '1', '1', t1.month - 1)

Answer 1

You can use LAG to see the previous row's cost.

select 
  year, user, month, type, cost, 
  cost - coalesce(lag(cost) over(partition by user order by year, month), 0) as costmtd, 
  0 as blank
from dbo.usercosts as
order by user, year, month;